This invention relates to drilling compositions and methods, and more particularly to drilling compositions useful in hydrocarbon well drilling operations which change from a more hydrated condition to a less hydrated condition in response to a change in an environmental condition.
During drilling of a well such as a well for extracting hydrocarbons, fluids are pumped into the well. Different fluids are used during different stages of the drilling operation. At one stage, fluids known as workover and completion fluids are used. Such fluids serve several purposes such as cooling and lubricating the drill bit, and transporting solids to the surface. Another important function of the drilling fluid is to balance the hydrostatic pressures encountered as the well is drilled. A condition called "blow-out" will occur if the pressure in an underground formation is greater than that provided by the fluid. In order to balance the pressures encountered in deep wells, fluids denser than water are typically required. One technique for increasing density is to create a brine or saturated salt solution which increases density up to 18 pounds per gallon.
To assure that a blow-out does not occur, the well is drilled in an over-balanced state, the pressure of the drilling fluid being higher than that encountered in the formation. Such over-balanced drilling, however, will cause the drilling fluid to infiltrate into the stratum which will damage the formation and greatly impair the ability to recover material from the well once production begins. Therefore, in the prior art, drilling fluids often include solids known as bridging agents to serve as a barrier to prevent the drilling fluid from infiltrating into the formation pore spaces and thereby damaging the formation. One prior art technique is to suspend salt crystals in an already saturated salt or brine solution to bridge the formation pore spaces to prevent formation damage. See, U.S. Pat. Nos. 4,186,803; 3,785,438; 3,878,141; 4,175,042; 4,369,843; and 4,822,500. Prior art bridging agents temporarily bridge over the formation pores rather than permanently plugging the pores. These bridging agents are dissolved when the workover or completion operation is accomplished to allow production from the well. A suspension agent is required to suspend the salt crystals but prior art suspension agents such as xanthan gum work only in monovalent ion solutions such as sodium chloride and sodium bromide. Divalent ion solutions such as CaCl.sub.2 and CaBr.sub.2 have distinct advantages over the monovalent ion solutions, such as NaCl and NaBr. For example, a CaCl.sub.2 solution can be made much denser than a NaCl solution while maintaining a solids-free fluid. Further, the CaCl.sub.2 is substantially cheaper than the NaBr at the same densities. The use of CaBr.sub.2 raises the maximum density to 14.3 lbs/gallon while maintaining a solids-free fluid. A drilling fluid that uses these brines as a base fluid has long been sought, but there has been no material that could be added that would have the necessary fluid-loss control and could be easily cleaned from the well.